The invention relates to a process for the thermal conversion of methane into hydrocarbons of higher molecular weight and the apparatus for carrying out the said process. More particularly, it relates to a process for the thermal conversion or cracking of the methane in a reactor comprising electric heating means and, by thermal dehydrogenation of this molecule, permitting the production principally of acetylene, ethylene, benzene and a little coke.
Any methane sources well known to a man skilled in the art may be used. Natural gas may cited as a highly current source of methane. A non-exhaustive list of these sources is provided for instance in European Patent Application EP-A-323287 in the name of the Institut Francais du Petrole, one of the assignees of this invention. In the majority of cases, the gas containing methane which is introduced into the reactor contains from 1 to 90% and sometimes even more of at least one other gas.
In European Patent Application EP-A-323287, there is described a process for the thermal conversion of methane into hydrocarbons of higher molecular weight, comprising electric heating means with a transfer of heat to the gaseous mixture containing the methane to be converted, through seal-tight walls of sheaths of ceramic material which insulate the said heating means from the gaseous mixture containing the methane. In this process, the heating zone is heated by a supply of electrical energy by means of electric resistors and the heat released by Joule's effect in these resistors is transmitted mainly by radiation to the sheaths of ceramic material disposed around resistors in a non-contiguous manner. The gaseous feeds which circulate substantially at right-angles to the axis of the heated sheaths are heated substantially by convection and by radiation. In the performance of this process, two spaces are defined within the reactor:
on the one hand, the reaction space or process space outside the sheaths protecting the resistors, in which the gaseous mixture containing methane circulates, PA1 on the other hand, the resistor space formed by the volume comprised between the actual resistors and the insulating sheaths into which preferably an inert gas, that is to say a gas containing no methane or any hydrocarbon capable of a thermal conversion reaction or any compound capable of reacting violently with methane or hydrogen, is introduced. This gas is likewise chosen in such a way that it does not damage the resistors used and does not cause accelerated ageing of the resistors. PA1 to have a relatively large quantity of hydrogen present in the process zone, PA1 to provide electrical resistors capable of delivering at high temperature a considerable quantity of energy per unit of surface area and per unit of time, PA1 to have conditions conducive to satisfactory heat transfer so that the temperature of the heating elements (that is to say the temperature of the surface of the sheaths in contact with the gaseous mixture containing methane) is not too much greater than the temperature desired for carrying out conversion of the methane. PA1 to minimize coke formation, particularly on hot surfaces such as for example the walls of sheaths enclosing the resistors, PA1 to use as a gas in the resistor space a gas or a mixture of gases preferably comprising a gas already present in the gaseous mixture circulating in the process space, which makes it possible to use sheaths which do not possess a very high level of seal-tightness, PA1 to improve heat exchange between the gaseous mixture containing the methane and the hot surfaces in contact with this mixture, PA1 to minimize the problems of distribution of the gases inside the reactor, PA1 to enhance the viability of the apparatus and its ease of construction and dismantling for decoking and maintenance of the reactor. PA1 they accept a considerable charge (power emitted per unit of surface area) which may be as much as 20 W/cm.sup.2, PA1 they can work at a very high temperature PA1 they display negligible ageing in course of time, PA1 they readily withstand reducing atmospheres at elevated temperatures. In the process according to the invention, the heating zone is followed by a cooling (or quenching) zone in order very rapidly to reduce the temperature of the effluent from the zone to approx. 300.degree. C. for example.
One of the greatest problems when carrying out the thermal conversion of methane is linked with coke formation. Indeed, if it forms in too great a quantity, it is likely to damage the furnace before the decoking operations can be performed and furthermore, from the economic point of view, its formation represents a substantial loss both with regard to the electrical energy consumed and the methane consumed in forming the coke. This problem, well known to a man skilled in the art, is partly resolved by she introduction into the gaseous mixture containing the methane to be converted of a quantity of hydrogen representing from 1 to 90% by volume in relation to the total volume of gas. In spite of this precaution, coke formation has not been completely eradicated, mainly at the level of the walls of the sheaths and on the other surfaces which are at elevated temperature and which are in contact with the gaseous mixture containing the methane.
This explains why, in carrying out the methane conversion process in an electrically heated pyrolysis furnaces, it is desirable:
In carrying out the process, it has been stipulated that it is preferable for the resistor space to be filled by a gaseous medium such as nitrogen, carbon dioxide or air. The use of air is only conceivable if the seal provided by the sheaths between the process space and the resistor space is perfect. Indeed, there would otherwise be a substantial risk of forming a gaseous mixture at a very high temperature, comprising oxygen, methane and hydrogen, which therefore entails a risk of explosion. The provision of a completely seal-tight system is relatively difficult and furthermore requires the use of ceramic material which offers a very high level of seal-tightness and which is therefore of very high quality, that is to say a ceramic material the density of which is close to theoretical density and which is free from open pores.
The use of such a ceramic material is extremely expensive, which penalizes the process. Therefore, one is induced to accepting the use of sheaths of less than perfect seal-tightness and to use either nitrogen with the not inconsiderable risk, in view of the resistor skin temperature, of the formation (in the case of silicon carbide resistors) of silicon nitride, which in principle has no effect on the mechanical strength of the resistors but does cause a fluctuation in the resistivity of these resistors and therefore accelerates their ageing, the more so the higher the temperature of the resistor (and therefore of the heating element) and the greater the amount of energy provided by the resistor, or to use carbon dioxide gas which, even if the rate of leakage from the resistor space into the process space is minimal, will inevitably cause problems at the stage involving separation of the products formed during the course of thermal conversion of the methane, complicating this stage on the one hand by their presence and on the other by the presence of carbon monoxide and water which will inevitably form by reaction between the carbon dioxide, the methane, the coke and the hydrogen in the process space.